nonuniaxial

Nonuniaxial is a term used to describe optical properties of materials that do not belong to the uniaxial class. In the context of optical crystallography and mineralogy, nonuniaxial materials are those that do not have a single optic axis. The most common nonuniaxial category is biaxial crystals, which possess two optic axes and three principal refractive indices. Some contexts also include optically isotropic materials under the broader umbrella, though these lack birefringence altogether and are considered isotropic rather than nonuniaxial.

In uniaxial crystals, light propagates along one optic axis without double refraction, while directions away from the axis produce two refracted rays with distinct indices. By contrast, biaxial (nonuniaxial) crystals have a more complex index structure, described by three principal refractive indices and an index ellipsoid with two optic axes. Along the two optic axes, birefringence vanishes and light travels with a single refractive index; in other directions, light splits into two rays with different speeds and polarizations.

The term is primarily used to distinguish nonuniaxial behavior from uniaxial behavior in optical mineralogy and crystallography. It emphasizes the presence of either two optic axes (in biaxial crystals) or the absence of a single axis of optical symmetry, reflecting a broader range of anisotropic optical properties compared with uniaxial materials.